Power units
.
W (watts) – Basic unit ofpower; assumes 8
Ω
(or user-settable)
load
.
dBm – Decibels relative to 1 milliwatt into 600
Ω
(or
user-settable) load
The Portable One Plus Access cannot directly measure power.
However, power can be calculated from amplitude and impedance (or
resistance), so ifthe value ofthe impedance is provided, a power
reading can be obtained from a voltmeter reading. Both of the power
units use this technique.
The Watts calculation is simply the voltage squared divided by the
given impedance.
Unfortunately, the dBm unit is still mistakenly used when the circuit
impedance is well over 600
Ω
. Modern equipment manufacturers may ask
that a +4 dBm signal be applied to a device for calibration when the
circuit impedance is 10 k
Ω
, which would result in a signal amplitude of5
Volts. This may be so large as to overload the input circuitry, causing it to
clip. What is really desired is a +4 dBu signal, which is 1.22 Volts. This is
a much more reasonable level for modern audio equipment.
So how can you tell which specifications are truly dBm and which are
dBu? As a general rule, you should read ‘dBm’ as ‘dBu’ unless you know
you are working with equipment terminated in 600
Ω
. Ifyou are using
600
Ω
equipment, you can use dBm units. You can easily measure the
input impedance ofa device using the Portable One Plus Access’s Gen
Load function.
Ifyou are testing equipment terminated in high impedances, or ifyou are
unsure ofthe input impedance ofthe device, it is best to select the
minimum impedance for the Portable One Plus Access generator and use
the dBu units. In most cases this will be correct, and even in the worst
case this will only be off by a fraction of a dB.
&
For
information
about the Gen
Load function,
see page 4-55.
4
Oper
ati
o
n
Operation
Units : Power units
4-74
Portable One Plus Access User's Manual
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